1. Field of the Invention
This invention relates to an electric hand tool and, more particularly, to an electric hand tool which is driven by a small motor through a planetary reduction gear mechanism and in which the motor is energized by nickel-cadmium type cells having each a relief valve for releasing gas when the cell is overcharged.
2. Description of the Prior Art
Electric hand tools such as electric hand drills which have been so far known include those of the dry cell driven type in which dry cells are used to energize a small driving motor with a chuck holder on its shaft to removably support a tool such as a drill. In such a type of electric hand tool, however, it is necessary to use a relatively big motor because not very much current may be drawn from the dry cells. Moreover, it requires at least UM1 type dry cells and hence a space sufficient to accommodate them. Therefore the diameter of the conventional electric hand tool can be no less than about 3.5 centimeter, which is too big for convenience of operation, especially operation with a single hand. Further more, the UM1 type dry cell has the characteristic that its voltage decreases very largely during the earlier stages of its use and then decreases gradually; therefore the speed of rotation of the tool also decreases largely during the earlier time of the use of the cell. Another disadvantage of the dry cell type electric hand tool is that it requires high maintenance cost because the UM1 type dry cell is wasted after it has been used.